MYRTLE BEACH, South Carolina (CNN) -- The top three Democratic presidential candidates face off in a Monday night debate in South Carolina, with the hearts and minds of African-American voters on the line.

Sens. Hillary Clinton of New York and Barack Obama of Illinois and former Sen. John Edwards of North Carolina share the stage at Myrtle Beach's Palace Theatre as the nation honors the Rev. Martin Luther King Jr. with a federal holiday.

The debate, put together by CNN and the Congressional Black Caucus Institute, comes five days before the Democratic primary in South Carolina, where almost half of the Democratic primary voters are African-Americans.

These voters will be crucial to the outcome of Saturday's primary in South Carolina. They now appear to be leaning heavily toward Obama, who if elected, would become the country's first black president.

Having a debate on Martin Luther King Jr. Day in South Carolina "is very fitting," said David Bohrman, CNN senior vice president and Washington bureau chief, who is executive producer of the debate. "Perhaps a debate on Martin Luther King Day in South Carolina should be made a must-stop on the road to the White House every four years."

A CNN/Opinion Research Corp. poll released Friday suggested that nearly 60 percent of black registered Democrats were backing Obama as the presidential nominee, with 31 percent supporting Clinton. That's a major shift from October, when African-Americans backed Clinton over Obama, 57 percent to 33 percent.

"There's been a huge shift among African-American Democrats from Clinton to Obama. African-American Democrats used to be reluctant to support Obama because they didn't think a black man could be elected. Then Obama won Iowa and nearly won New Hampshire. Now they believe," Bill Schneider, CNN's senior political analyst, said.

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The Democrats face their toughest questions yet at a debate by the Congressional Black Caucus Institute and CNN.Tonight, 8 ET

"Obama's lead over Clinton among black men is more than 50 points, and among black women, once a Clinton stronghold, Obama has an 11-point advantage," said Keating Holland, CNN's polling director.

Entrance polls from Saturday's Nevada Democratic caucuses match what appears in the CNN poll. Eighty-three percent of black voters questioned before they entered the caucuses said they were backing Obama, with 14 percent supporting Clinton, who if elected, would become the country's first woman to win the presidency.

These kinds of numbers could spell trouble in South Carolina for Clinton, who's coming off victories in the New Hampshire primary and the Nevada caucuses. But Clinton holds a special relationship with many in the black community, thanks to her efforts in support of civil rights and to the popularity of former President Clinton with African-Americans.

Both Democratic front-runners were reaching out to African-American voters Sunday. Obama, who often refers to King in his speeches, spoke at Atlanta, Georgia's Ebenezer Baptist Church, where the late civil rights leader once preached. Watch Clinton, Obama reach out to black voters »

Obama recalled the legacy of discrimination against African-Americans but challenged the audience at the historic black church to take a look at a few lingering prejudices among some within the community.

"And yet, if we are honest with ourselves, we must admit that none of our hands are entirely clean. If we're honest with ourselves, we'll acknowledge that our own community has not always been true to King's vision of a beloved community," Obama said, citing homophobia, anti-Semitism and anti-immigrant sentiment.

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Obama told the congregation Sunday morning that if King could forgive his jailers, "surely we can look past what divides us in our time."

Obama's visit to the city coincided with his endorsement Sunday by The Atlanta Journal-Constitution, which cited his "appeal across many of the lines that have divided America," adding that "both Clinton and Obama would make very good presidents, [but] Obama is the person; this is his time."

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In New York, Hillary Clinton spent Sunday morning at another historic black church, Harlem's Abyssinian Baptist Church, where she picked up the endorsement of its pastor, the Rev. Calvin Butts.

Butts said outside the church: "A vote for Hillary Clinton is a vote to elect someone who has proven through time to me and to this community and this country that she has the experience to make things happen, and the vision to return us to a place of prosperity."

Clinton also sounded a conciliatory note Sunday. "I have the highest regard and admiration for my friend and colleague Sen. Barack Obama. I am honored to be running with him," she said. "I hope that this election remains focused on the big challenges that confront us."

After a distant third-place finish in the Nevada caucuses, Edwards on Sunday made light of his performance. On CNN's "Late Edition With Wolf Blitzer," Edwards said he hopes "what happens in Vegas stays in Vegas," admitting, "I got my butt kicked."

Edwards would like to rebound in South Carolina, his native state. He won the primary there in 2004 when he was making his first bid for the White House.

But he's running a distant third in most recent surveys in South Carolina, behind Obama and Clinton. On Sunday, Edwards sounded a cautious note, saying South Carolina was important but just one "part of the long process. ... We will see how it goes."

Also on "Late Edition," the House majority whip, Rep. James Clyburn, D-South Carolina, cited the timing of the debate to help put the spotlight on issues important to blacks. Clyburn, the highest ranking African-American in Congress, was instrumental in having Monday's debate held in South Carolina.

"When we were dealing with the dates of the primary, we tried to work in the symbolism that it would have to all of the world, for that matter, to have this debate on Martin Luther King Jr.'s birthday celebration," Clyburn told Blitzer, who is the moderator of the event.E-mail to a friend